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Personally, I would go with 2x4s unless you are very good with wood and possibly 2x6s for the cross beams. I used 2x4s for a 100 gallon, but I wish i made the doors bigger with a larger opening in the front, would have used 2x6s if i had to do it again.
They make them out of 1x4's because they don't want the stand to be able to hold much more than it is advertised for. Making a DIY stand with thicker lumber will give you the flexibility to use it for tanks that may have the same dimensions, but varying amounts of capacity.